High speed pulse processing

ABSTRACT

Method and apparatus for first converting energy pulses, such as from the energy-dispersive detector of an X-ray fluorescence spectrometer, into multiple ramp signals, the ramps of which are parallel with the vertical distance between the ramps being proportional to energy pulse height. Digital samples of the ramps are taken and, from the known slope of the ramps, the vertical distance or difference may be determined using a least-squares algorithm. This value is the absolute amplitude pulse value.

United States Patent 1 [111 3,928,766

Clausen et al. [45] Dec. 23, 1975 HIGH SPEED PULSE PROCESSING formanceby R. L. Heath et al. from Nucleonics,

[75] Inventors: Kermit D. Clausen; John R. Rhodes, 1966 52 56' both ofAustin, Tex.

[ 73] Assignee: Columbia Scientific Industries, Inc., PrimaryExaminer-James W. Lawrence Austin, Tex. Assistant ExaminerB. C. Anderson[22] Filed: Aug. 6, 1973 Attorney, Agent, or Firm-Arnold, White & Durkee[21] Appl. No.: 386,100

57 ABSTRACT [52] US. Cl 250/273; 250/272 1 [51] Int. Cl. G21K 1/00Method and apparatus for first converting energy pulses, such as fromthe energy-dispersive detector of an X-ray fluorescence spectrometer,into multiple ramp signals, the ramps of which are parallel with the[58] Field of Search ..235/l5l.35; 250/272, 273, 250/307, 310, 355, 370,371, 408, 409

[56] References Cited vertical distance between the ramps being propor-UNITED STATES PATENTS tional to energy pulse height. Digital samples ofthe 2,962,594 11/1960 Duffy 250/409 ramps are taken and, from the knownslope of the 3,433,954 3/ 1969 Bowman et al. ramps, the verticaldistance or difference may be de- 356794O 3/1971 Lambe" terrnined usinga least-squares algorithm. This value is 3,746,862 7/1973 Lombardo etal. 250/409 h absolute amplitude pulse valuev OTHER PUBLICATIONSDesigning Semiconductor Systems for Optimum Per- 11 Claims, 6 DrawingFigures 70 SPECIMEN X RAYS t/COMPTON SCATTER COHERE N T SCAT TER "iFLuoREscE/vcs ENERGY 12 3o 31 4o ANTl-AL/AS/NG A-T0-D DIG/ML D VETJECTOR PRJE AMP AMPLIFIER CONVERER PROCESSOR 14 w 22 FAST CONTROLCHANNEL LOG/C US. Patent Dec.23, 1975 Sheet 1 of3 3,928,766

mmk .GGW 20k @200 ZMEBMQW U.S. Patent Dec. 23, 1975 Sheet 2 of 33,928,766

FAST CHANNEL HIGH SPEED PULSEPROCESSING BACKGROUND or THE INVENTIONdetectors to a level where they may be processed to provide the requiredmeasurement data. The useful information in a pulse is usually containedin its amplitude and in its frequency (if periodic) or rate of arrival(if aperiodic). Random noise is always present. In one class ofinstruments (X-ray, gamma-ray and nuclear spectrometers), the rate ofarrival of signal pulses can be very high (for example, 10 to 10 pulsesper second) and is usually random. The pulses have different heights andthe output of the instrument is a histogram of the number of pulses in agiven heightinterval against pulse height. The resolution of thedetector (e.g., a solid-state detector) can be such that the width of auseful pulse height interval must be less than 0.1% of the maximum pulseheight being processed.

The conventional electronic techniques for amplifying these pulses withrespect to detector speed have been painfully slow and far below thecapacity of certain types of detectors (e.g., a solid-state'detector) toproduce meaningful energy charges. Attempts to speed up suchamplification have resulted in distorting the pulses significantly withrespect to detector resolution. Present solid-state radiation detectorshave nanosecond response times. State-of-the-art filtering systems foranalyzing charges develop analog pulses having durations on the order of60-70 microseconds. Faster filtering is not possible without introducingsignificant noise. The dead time existing in this pulse period is themajor limiting factor in the processing of pulses in the solid-stateradiation spectrometer and such period cannot be materially reduced evenwith the best possible analog filter. Hence, a fundamentally newapproach to treating pulses is needed to improve the operating speed.The theory, to be hereinafter explained more fully, does away withattempting to improve the filtering of an analog pulse. Instead, thetheory provides for deducing the peak value from certain sampled dataleading up to the peak and additional sampled data following the peak,all of which is treated digitally,

thereby avoiding having to use an analog filter.

Therefore, it is a feature of this invention to provide an improvedmethod of processing charges resulting from an energy-dispersive,solid-state detector so that the operating speed is superior to thatachievable by analog filtering for the same resolution.

It is still another feature of this invention to provide It is yetanother feature of. this invention to provide an improved system ofprocessing rapidly occurring pulses in the context of anenergy-dispersive, solid-state,

dead-time detector by using a pulsed source whose switch-on andswitch-off times arecontrolled by the detector elec tronics toessentially eliminate the dead-time portion of the periods of the pulsesbeing processed.

SUMMARY OF THE INVENTION A preferred embodiment of the present inventioncomprises, in an X-ray fluorescence spectroscopy apparatus, anenergy-dispersive detector for receiving the photon emissions from asample under analysis which is irradiated by a pulsed X-ray excitationsource. A preamplifier is connected to the detector for producingsawtooth voltage pulses proportional to the detector charges. The rampportions of these pulses are essentially parallel. A dc coupledamplifier amplifies the sawtooth signals to a measurable level which arethen supplied to an ADC network. Here, the sawtooth ramp portions aresampled at a clock rate to produce digital values at a plurality ofpoints along the ramp, thereby defining a ramp in digital form. Thedigital format of at least one of two adjacent ramps is projected intime so that the vertical distance therebetween is determinable. Thisvalue is a measure of the amplitude of a pulse and hence the value of apoint useful with a plurality of subsequent pulse value points in thedevelopment of an energy histogram. Such a histogram provides the meansfor determining the constituent parts of the irradiated sample in amanner well-known in the art. However, new is the development of thepulse value in the above manner and its processing to permit more rapiddevelopment of the histogram points than ever before, thus minimizingdead-time losses, without sacrificing resolution, and avoiding having tocorrect for baseline drift.

BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which theabove-recited features, advantages and objects of the invention, as wellas others which will become apparent, are attained and can be understoodin detail, more particular description of the invention brieflysummarized above may be had by reference to the embodiment thereof whichis illustrated in the appended drawings, which drawings form a part ofthis specification. It is to be noted, however, that the appendeddrawings illustrate only a typical embodiment of the invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

In the drawings:

FIG. 1 is a block diagram of a preferred embodiment of a systemoperating in accordance with the present invention.

FIG. 2 is a simplified schematic of apreamplifier suitable for operationin the system disclosed in FIG. 1.

FIG. 3 is a sample waveform developed in the embodiment of the inventionshown in FIG. 1 and a timing diagram with respect to operational pulses.

FIG. 4 are theoretical waveshapes useful in explaining the theory ofoperation for the present invention.

FIG. 5 is an expanded version of one of the waveshapes shown in FIG. 4,for detail analysis.

FIG. 6 is a simplified block diagram of the digital processor used in apreferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT A new method of signal processing isrequired over the prior art in order to overcome the signal processingproblems attendant in the prior art methods. The signal to be amplifiedmay be thought of as the sum of random noise E(t) and the informationsignal v(t). The infor- 3 mation signal is further assumed to be theproduct of a constant-amplitude periodic function v,,(t) and anamplitude modulating function a(t) where s a s 1. Hence, the methoddisclosed herein may be thought of as a method of processing V(t) v(t)E(t) v,,(t) -a(t) E(t) much more rapidly than before possible and henceto obtain an accurate measure of a(r) for each pulse.

Certain limits may be placed on v,,(t) and a(r) to agree with thewaveshapes as they appear in the embodiment of the invention hereinafterdisclosed. For this discussion, refer to FIG. 4. First, v,,(t) isassumed to be a sawtooth or triangular function. Second, a(t) is assumedto be constant for any given value from one period of v,,(t) to thenext. Hence, a(t) is assumed only to affect the height of the triangularpulse and not its slope, which remains constant. Thus, it may be seenfrom observing the waveshape of v(t) that all of the information in thepulse is contained in its relative height.

The determination of the height of a pulse of V(t) may be understoodwith reference to FIG. 5. The pulse has a period T and is sampled Ntimes per pulse.

From the time and amplitude sample pairs, a line of best fit using thestandard least squares method is obtained. To measure the maximum heightof V(t) at time 1 X 10 sec, called V'(t), the following expressionapplies:

V'u) A B(: T), where B is the known slope;

l A EV..

t'= average time over which the samples are taken. In terms of FIG. 5values, for example,

B is known to be equal to 0.8/] X The value of (t-f) equals 0.5 X 10Hence,

The minimum point at time 2 X 10 may be calculated from the expression Vr) A B (T-t).

A similar substitution of numbers shows that the amplitude of this valueis equal to 0.2. For the next pulse, the maximum height of V(t) at 2 X10 may be similarly calculated to be 1.05. Then, the height of thedifference, the value of a(t), is equal to 1.05 0.2 or 0.85.

Since the slope and period are known, E(t) will only enter thecalculation through A. Since the average value is equal to zero (noisebeing normally randomly distributed), it may be shown that foruncorrelated noise the variance in the calculated height is inverselyproportional to the number of samples. Hence, although it is possible totheoretically obtain results as above with only one sample per line, itis necessary to take several to minimize the noise variance influence.

The above description will be helpful in understanding the method usedin the description of the overall system, since the signal which isoperated on has a waveform that closely resembles that which is shown inFIGS. 4 and 5.

Now referring to FIG. 1, a block diagram is shown of an X-rayfluorescence spectroscopy apparatus. An X-ray source 10 is suitablypositioned for bombarding a sample or specimen l2 material underexamination with low energy photons to ionize the constituent atoms ofthe sample. Not uncommonly, a physical arrangement includes a secondarytarget interposed between the X-ray source proper and the sample, butsuch arrangement is not required and is not illustrated.

Sample 12 may take various forms, but it is commonly a thin deposit ofmaterial under analysis prepared on a clean filter disk. Samplepreparation and the physical set-up of typical X-ray fluorescenceapparatus is described, for instance, in Energy Dispersion X-RayAnalysis: X-Ray and Electron Probe Analysis, Special Publication STP485, American Society for Testing and Materials, Philadelphia, 1971 andThe Si(Li) X-Ray Energy Analysis System: Operating Principles andPerformance, D. A. Gedcke, X-Ray Spectrometry, Vol. 1, page 129, 1972.

As explained in the latter article, the advent and refinement of theSi(Li-drifted) or Si(Li) detector diode as a detector for X-rayspectroscopy has spawned a new generation of fluorescence analyzers. TheSi(Li) detector may be described as an energy-dispersive detector thatconverts X-ray photon energy to an electrical charge.

In simple terms, when sample 12 is irradiated or bombarded with X-raysfrom the source, excitation in the sample varies according to itsconstituentparts. Hence, the photon emission from the sample to thedetector includes photon energies statistically dependent on thequantity of the constituents in the sample. That is, each time the sampeis irradiated, an interaction occurs within the Si(Li) detector thatproduces an ionization cloud, which is swept from the detector by a biasvoltage, in the form of a charge. Over a period of time, these charges,each being referred to as a charge event or merely as an event, will beat various levels and, as will be hereafter explained, can beelectronically treated to form a histogram of the number of X-raysdetected versus energy. From this histogram,

constituent makeup of the sample can be determined.

One of the principal advantages of the Si(Li) energydispersivespectrometer over the wavelength spectrometer is that quick andsimultaneous analysis of all elements with atomic numbers equal to orgreater than 11 may be obtained.

Among the many other advantages of the energy-dispersive system usingthe Si(Li) detectors, besides its main ability to analyze the nearlyentire spectrum at once, is its high detection efficiency, itscompactness, its excellent stability (no moving parts), and the absenceof an optical focusing requirement. Although the prior art Si(Li)detectors have opened up a new generation of X-ray fluorescenceapparatus, amplifier circuits used heretofore to filter and shape thebasic charge have introduced dead time periods of relatively longduration compared with the time for a charge to be converted to a peakvoltage value (and hence to a potentially detectable and measurablequantity that can be electronically processed).

To more fully appreciate the limitations in the prior art apparatus, afuller explanation of such apparatus is in order. In such a prior artsystem, the charge from the detector is applied to a charge-sensitivepreamplifier that produces an output voltage pulsehaving an amplitudeproportional to the X-ray' energy. Even the most sophisticatedpreamplifiers add noise, one of the limiting factors in the system. I 1

The signal is coupled to afilter-amplifier to amplify the preamplifieroutput signal to a measurable level for a multi-channel pulse heightanalyzer. The amplifier includes shaping circuits to filter the pulsesignal into a near Gaussian signal; which is peak detected in ananalog-to-digital converter (ADC) and then converted into a numberproportional to the peak amplitude of the amplifiers output signal. Thismeasurement is repeated on a pulse-by-pulse basis to build up the energyspectrum as a histogram.

During the time the amplifier is filtering a signal the system undergoesdead time, since there is no way of accommodating another informationcharge from the detector. These dead-time loss periods are long and manymore charges are not used compared with those that are used forinformation processing. For example, it takes about microseconds for atypical CR-RC filtering network to respond to a charge and produce apeak voltage output that is proportional thereto. If this value is r,the time constant for a CR-RC filter, the overall time for the value toreturn to 0.1% of its peak value (near zero) is typically about 115microseconds,

- or the overall base-to-base time for the pulse is about 12.5 T. Evenfor the most sophisticated multi-stage filters including multiple stagesof integrators, the filtering time is still 50-60 microseconds.

In the system described herein, a pulsed source is employed. With apulsed source, the concept of dead time is changed. As such, there is nodead time in the conventional sense. There is, however, a pulseprocessing time which establishes the upper limit for the speed of pulseprocessing. As will be hereafter shown, this new concept in pulseprocessing achieves a substantial improvement over the conventionalsystem of pulse handling. Further, the method of pulse-height measuringalso minimizes the limiting effects of noise insertion by thepreamplifier.

Now returning to FIG. 1, the charges from specimem 12 is received bydetector 14, as'with the prior art systems. The output of the detectoris, in turn, electrically connected to charge-sensitive preamplifier 16.

Prior art preamplifiers may be thought of as merely including anoperational amplifier having a parallel RC circuit connecting its outputand input. Although this type of preamplifier successfully performed therequired integration step, the RC circuit establishing the time constantfor the operation, such a circuit also causes an undesirable dc build-upat the output at a high pulse rate that results in the preamplifieroperating in a non-linear region. 1 i

Referring to FIG. 2, a block diagram of a preamplifier circuit inaccordance with the present invention is shown that uses a long timeconstant network, but which minimizes dc build-up. An operationalamplifier 18 is connected in the conventional manner, with capacitor C,connected from its output to input. An amplifier 20 is connected inseries with resistor R the series combination connecting the outputofoperational amplifier 18 with its input. The ac component of i theoutput is equal to q,/C,, q, being th'e'charge released by solid-statedetector 14 for a given interaction. The

voltageno longer affects system performance since amplifier 18' onlyhandles the ac part of the pulse. The,

time constant of QR; may be made quite large, to make the output atVgvirtually'linear in the region where it is important for processingpurposes. This produces a sawtooth or triangular waveform with nearlystraightline decay sloping sides. The output pulse height V isproportional to the charge delivered to the input from the detector.

Returning to FIG. 1, the output of the preamplifier is applied toananti-aliasing or low-pass filter comprising in-line resistor 22 andcapacitor 24 to ground. The anti-aliasing filter removes the highfrequency components that would otherwise interfere in the results ofdigital processing. The filtering bandwidth is set for about half thesampling frequency. The preamplifier output is also applied to fastchannel circuit 26, which produces a trigger signal to control logiccircuit 28 each time it detects an output event from preamplifier 16.The purpose of the development of this triggering signal will behereafter explained.

The sawtooth signal from the preamplifier is supplied through theanti-aliasing circuit to dc amplifier 30. This amplifier amplifies thepreamplifier signal to a measurable level. The output from amplifier 30is supplied to analog-to-digital converter (ADC) 31. The technique ofmeasuring the amplitude of this signal may best be understood byreferring to FIG. 3.

FIG. 3 shows an expanded representation of a typical pulse sequence fromamplifier 30 as supplied to ADC 31. Because of the large time constantvalue of the components in the preamplifier, the output resulting fromthe detected charge is a virtually linear voltage ramp 32. Ramp 32 issampled at a plurality of points- 32a, 32b, 32c, and 32d, the samplingtimes being determined by clock pulses occurring at regular intervals.The generation and use of clock pulses is well known in the art. After apredetermined number of samples have been taken (in the exampleillustrated, four samples are taken), a trigger pulse is produced thatcauses X-ray source 10 to generate another X-ray from the sample orspecimen 12, which, in turn, results in a voltage from the preamplifier.The rise time for this voltage output is a function of the collectiontime for the energy charge to accumulate, which, in turn, is a functionof the solid state properties of the silicon in the Si(Li) diodedetector.

The maximum or peak output from the preamplifier is not sensed, but itis known that after a predetermined period of time the output will againbe producing a decaying ramp voltage 34. Again, as with ramp voltage 32,ramp voltage 34 is sampled at a plurality of points 34a, 34b, 34c and34d at regular intervals determined by the clock pulses. Notice thatthese sample pulses have been suspended for a predetermined number ofpulses before sampling is resumed, the amount of suspended sampling timefor illustration purposes being sufficient to accommodate four pulses.The time between suspension of sampling and resumption must besufficient'to ensure that the output of preamplifier 16 isproducing'ramp 34 and is not on the rise portion between ramps. Also,notice that ramps 32 and 34 are substantially parallel.

A measure of the pulse amplitude is vertical distance 38 betweenparallel ramp 32 (or more accurately, its straight line extension 36)and ramp 34. Actually, since ramps 32 and 34 do not occur simultaneouslyin time, it is necessary to project the value of extension 36 from 7sampled values 32a, 32b, 32c, and 32d. Therefore, when value 34a isdetermined, difference value or vertical distance 38 may be determined.

Again referring to FIG. 1, the clock pulse generator and the logiccircuitry for producing the sample pulses are included in control logiccircuit 28. Control logic circuit receives an input from fast channelcircuit 26 and produces outputs to X-ray source 10, ADC 31, and digitalprocessor 40.

Initially, there is an output from control logic circuit 28 to turn onX-ray source 10. This pulse to source 10 may be assumed to occur at thesame time as the last of the sample pulses in a sampling series suppliedto ADC 31. There is then a suspension of an output from control logiccircuit 28 to ADC 31 for the number of counts included in the suspensionperiod between the sampling series. Then, sampling pulses are againproduced to ADC 31.

There is an output triggering pulse produced from fast channel circuit26 each time this circuit detects an event from preamplifier 16. It maybe recalled that when preamplifier receives a charge from detector 14,it produces an output which is the eventual subject of analysis. But,this output is also useful as an initiating signal to fast channelcircuit 26, the output of which is supplied for convenience throughcontrol logic circuit 28 to shut off X-ray source 10. It is immaterialthat the energy level differs from pulse to pulse or that at this pointthe preamplifier output has not been filtered. What is important is itsexistence, which is all that is required to produce an output triggeringpulse from the fast channel circuit. Hence, only one charge at a time isproduced for analysis. X-ray source 10 is held off until the nextinitiating signal from control logic 28.

Although what has been described above is absolutely controlled by theclock pulse rate, it is alternatively possible to initiate eachsuspension and sampling series with the output from fast channel circuit26. When this is done, then the time between the last pulse in thesampling series and the fast channel pulse (see FIG. 3) has to bemeasured since this time enters into the calculated projected value ofextension 36.

The network in ADC 31 which is primarily instrumental in converting theanalog signal occurring at the time of sampling to a digital value isreferred to as a sample-and-hold network. Actually, this circuit,wellknown in the art, follows or tracks an analog voltage and at eachsampling occurrence produces a dc output at the current analog level.This output persists from the network until the next samplingoccurrence. Hence, the analog signal is converted to a stairstep signal,or a plurality of successive dc voltage values. It is again well-knownhow to convert a dc voltage to a digital count. A 12-bit output resultsin the capability of producing well over 1000 different channels for thehistogram. Notice that such range is possible even though there are veryfew samples taken. Hence, the processing time may be kept quite shortcompared with prior art peak detector processing systems and the like.In truth, the analog signal is converted to a digital format, which canbe rapidly manipulated by well-known digital circuits. Thus, stretchingthe art of analog processing is avoided altogether.

The operation of digital processor 40 may be best understood by firstunderstanding the development of the operating algorithm. The problem isto develop the value of vertical distance 38. For convenience, thisquantity may be designated Y, the incremental points on ramp 34 wherethe measurements are taken may be designated Y and the incrementalpoints on the previous ramp 32 where the measurements are taken may bedesignated Y The formula for the difference in Y values, in generalterms, may be shown to be as follows:

Y2 Y1 2 1) M r 2). wherein l l t W n. 2 2h b slope and f, and 55 are theaverage time values to which Y and Y respectively pertain. Since slope,and f X can be assumed to be constant, the formula reduces to thefollowing:

It is this formula which is acted on by digital processor 40.

An expanded simplified block diagram of an embodiment of digitalprocessor 40 is shown in FIG. 6. The incremental Y,- values are suppliedto N bit shift register 42. At first these are the incremental valuesalong ramp 32 and then they are, after the suspension period, theincremental values along ramp 34. The logic circuitry for stepping downthe values along ramp extension 36 is included in the register.

The output and the input to register 42 are supplied to subtractor 44,where the incremental difference values are developed. The output ofsubtractor 44 is supplied to adder 46, which supplies its output toaccumulator 48. The output of accumulator 48 is supplied as anotherinput to adder 46, so that the output from the accumulator is asummation of the incremental vertical differences. The accumulator isconnected to another adder 50, to which is also supplied the constant Kinformation multiplied by the number in the sampling series. In theexample illustrated, this number is 4. Finally, adder 50 supplies itsoutput to divide-by-N circuit 52 to produce the output or verticaldistance 38 (value Y) to the multichannel analyzer where the histogramis developed in a manner well-known in the art.

For simplicity, reset connections and clock pulse connections are notshown in FIG. 6. Operation is successive only for the number of samplingpulses and is suspended during the suspension period between samplingseries. Conventional gating circuits may be used for this purpose.

The development of the slope information may be achieved in variousways. A separate run for each particular specimen may be made overseveral events, the slope being determined from the measured points. Theslope is a function of count rate and not just the values of C,R, andtherefore must be separately determined for each specimen. Then, forthis particular specimen, the value may be supplied as a function of theNK" input to adder 50. Alternatively, it can be developed directly fromthe V value of the preamplifier shown in FIG. 2 via sample-and-hold andADC circuits not shown, but which are similar to those used in ADC 31.The value N" for the number of samples in a series must also besupplied.

It is well-known that for several reasons, a very large number of pointshave to be plotted to make a reliable histogram. The large number ofchannels of energy 9 level separation (typically, 1024) means that ittakes awhile for a number of points in each channel tobe developed.About a minimum of 300 points is necessary before the data is consideredvalid. Further, many more points are collected in the coherent andCompton backscatter regions than in the fluorescence energy regionswhere meaningful analysis can be performed. Still further, each peak isrepresented by at least -10 channels, again requiring a very largenumber of data points. The saving in time of the system just describedover the prior art to obtain a nominal 1 million data points for thehistogram through the elimination of the dead time losses previouslyexisting is one of the great advances of the present system over theprior art.

While a particular embodiment of the invention has been shown, it willbe understood that the invention is not limited thereto, since manymodifications may be made and will become apparent to those skilled inthe art.

What is claimed is:

1. Method of processing energy charges from an energy-dispersivedetector to determine the amplitude values of said charges, the detectoroperating in an emission spectroscopy system wherein a specimen underanalysis is illuminated by a pulsable source capable of excitingcharacteristic X-rays, which comprises:

detecting a first energy charge with a long time constant circuit, theoutput therefrom being a first substantially linear decay; sampling theamplitude value of said first linear decay and converting said samplevalue to digital form;

creating a second energy charge for detection by the detector by pulsingon said source, the output from the detector being a secondsubstantially linear decay;

at a measurable time thereafter, sampling the amplitude value of saidsecond linear decay and converting said sampled value to digital form;

digitally projecting along the slope of said first linear decay theamplitude value at the time of the sampling of said second linear decay;and

subtracting the value of said projected first linear decay from saidsampled second linear decay to determine the relative amplitude value ofsaid first and second decays.

2. The method as set forth in claim 1, including sampling said first andsecond linear decays at a predetermined plurality of points so as todevelop a plurality of relative amplitude values of said first andsecond charges, and the additional step of determining the averagerelative amplitude value from said plurality of relative amplitudevalues of said first and second charges.

3. Method of processing energy charges from an energy-dispersivedetector to determine the amplitude values of said charges, the detectoroperating in an emission spectroscopy system wherein a specimen underanalysis is illuminated by a pulsable source capable of excitingcharacteristic X-rays, which comprises:

detecting a first energy charge with a long time constant circuit, theoutput therefrom being a first substantially linear decay;

sampling the amplitude value of said first linear decay and convertingsaid sample value to digital form;

creating a second energy charge for detection by the detector by pulsingon said source, the output from 10 the detector being a secondsubstantially linear decay; h at a measurabletime thereafter, samplingthe amplitude value of said second linear decay and convertingsaidsampled value to digital form; determining the slope of at least one ofsaid first and second linear decays; digitallyproje'ct'ing along saiddetermined slope one of said first and second linear decays to determinethe amplitude value thereof at the time of the nonprojected other ofsaid first and second linear decays; and subtracting the value of saidprojected decay from said non-projected decay to determine the relativeamplitude value of said first and second decays. 4. X-ray fluorescencespectroscopy apparatus, comprising:

an X-ray excitation source for bombarding a sample to cause photonemission of electromagnetic radiation characteristic of the constituentelements of the sample; an energy dispersive detector for receiving thephoton emissions from the sample, said detector establishing chargesproportional to the energy of successively received X-ray photons, saiddetector being sufficiently energy sensitive for the detection of photonenergy levels representative of at least 10 adjacent elements; amplifiermeans having substantially linear amplification characteristics forproducing sawtooth output voltage pulses proportional to the chargesfrom said detector, the ramp portions of the sawtooth signal from saidamplifier decaying in nearly linear fashion at substantially parallelslopes; fast triggering means activated by an output from said amplifierfor turning off said X-ray source after a single output from saiddetector; ADC means connected to said amplifier; logic means causingsaid ADC- means to sample the amplitude values of the output from saidamplifier at a plurality of points along a first decaying ramp portionof said sawtooth signal, said logic means turning on said X-ray sourceafter a predetermined number of samples have been taken;

said logic means causing said ADC means to sample the amplitude valuesof the output from said amplifier at a plurality of points along thenext subsequent decaying ramp portion of said amplifier output followinga suspension of sampling for a measured period of time; and

digital processing means for projecting the values of said firstdecaying ramp along an extension thereof to a point in timecorresponding with the sampling of said next subsequent decaying rampportion and determining the amplitude difference value therebetween.

5. X-ray fluorescence spectroscopy apparatus as set forth in claim 4,wherein said detector includes a Si(Li) detector diode.

6. X-ray fluorescence spectroscopy apparatus as set forth in claim 4,wherein said logic means produces a first series of clock pulses thatcauses the ADC means to sample said first decaying ramp portion, and,after a measured period of time, produces a second series of clockpulses that causes the ADC means to sample said subsequent decaying rampportion.

12 ing ramp and said next subsequent decaying ramp at a plurality ofpoints corresponding to the sample points and developing the averagevalue thereof.

10. The method as set forth in claim 1, wherein the emissionspectroscopy system is an X-ray fluorescence system.

11. The method as set forth in claim 3, wherein the emissionspectroscopy system is an X-ray fluorescence system.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Q PATENT NO.3,928,766

DATED December 23, 1975 |NVENTOR(S) Kermit D. Clausen; John R. Rhodes Itis certified that error appears in the above-identified patent and thatsaid Letters Patent Q are hereby corrected as shown below:

Col. 3, line 46, "v' (t) .6 should read 8 -5 -V'(t) .6 (5 x 10 1.0-- Q 1x 10- Col. 5, line 64, "q /C ,q should read q /C ,q

Col. 5, line 68, "R /I should read -R I Col. 5, line 68, "I should read-I Signed and Scaled this Twelfth D f October 1976 [SEAL] Y Arrest:

RUTH C. MASON C. MARSHALL DANN A! I P811718 ff Commissioner nj'Parenrsand Trademarks

1. Method of processing energy charges from an energy-dispersivedetector to determine the amplitude values of said charges, the detectoroperating in an emission spectroscopy system wherein a specimen underanalysis is illuminated by a pulsable source capable of excitingcharacteristic X-rays, which comprises: detecting a first energy chargewith a long time constant circuit, the output therefrom being a firstsubstantially linear decay; sampling the amplitude value of said firstlinear decay and converting said sample value to digital form; creatinga second energy charge for detection by the detector by pulsing on saidsource, the output from the detector being a second substantially lineardecay; at a measurable time thereafter, sampling the amplitude value ofsaid second linear decay and converting said sampled value to digitalform; digitally projecting along the slope of said first linear decaythe amplitude value at the time of the sampling of said second lineardecay; and subtracting the value of saId projected first linear decayfrom said sampled second linear decay to determine the relativeamplitude value of said first and second decays.
 2. The method as setforth in claim 1, including sampling said first and second linear decaysat a predetermined plurality of points so as to develop a plurality ofrelative amplitude values of said first and second charges, and theadditional step of determining the average relative amplitude value fromsaid plurality of relative amplitude values of said first and secondcharges.
 3. Method of processing energy charges from anenergy-dispersive detector to determine the amplitude values of saidcharges, the detector operating in an emission spectroscopy systemwherein a specimen under analysis is illuminated by a pulsable sourcecapable of exciting characteristic X-rays, which comprises: detecting afirst energy charge with a long time constant circuit, the outputtherefrom being a first substantially linear decay; sampling theamplitude value of said first linear decay and converting said samplevalue to digital form; creating a second energy charge for detection bythe detector by pulsing on said source, the output from the detectorbeing a second substantially linear decay; at a measurable timethereafter, sampling the amplitude value of said second linear decay andconverting said sampled value to digital form; determining the slope ofat least one of said first and second linear decays; digitallyprojecting along said determined slope one of said first and secondlinear decays to determine the amplitude value thereof at the time ofthe non-projected other of said first and second linear decays; andsubtracting the value of said projected decay from said non-projecteddecay to determine the relative amplitude value of said first and seconddecays.
 4. X-ray fluorescence spectroscopy apparatus, comprising: anX-ray excitation source for bombarding a sample to cause photon emissionof electromagnetic radiation characteristic of the constituent elementsof the sample; an energy dispersive detector for receiving the photonemissions from the sample, said detector establishing chargesproportional to the energy of successively received X-ray photons, saiddetector being sufficiently energy sensitive for the detection of photonenergy levels representative of at least 10 adjacent elements; amplifiermeans having substantially linear amplification characteristics forproducing sawtooth output voltage pulses proportional to the chargesfrom said detector, the ramp portions of the sawtooth signal from saidamplifier decaying in nearly linear fashion at substantially parallelslopes; fast triggering means activated by an output from said amplifierfor turning off said X-ray source after a single output from saiddetector; ADC means connected to said amplifier; logic means causingsaid ADC means to sample the amplitude values of the output from saidamplifier at a plurality of points along a first decaying ramp portionof said sawtooth signal, said logic means turning on said X-ray sourceafter a predetermined number of samples have been taken; said logicmeans causing said ADC means to sample the amplitude values of theoutput from said amplifier at a plurality of points along the nextsubsequent decaying ramp portion of said amplifier output following asuspension of sampling for a measured period of time; and digitalprocessing means for projecting the values of said first decaying rampalong an extension thereof to a point in time corresponding with thesampling of said next subsequent decaying ramp portion and determiningthe amplitude difference value therebetween.
 5. X-ray fluorescencespectroscopy apparatus as set forth in claim 4, wherein said detectorincludes a Si(Li) detector diode.
 6. X-ray fluorescence spectroscopyapparatus as set forth in claim 4, wherein said logic means produces afirst series of clock pulses that causes the ADC means to sample saidfirst decaying ramp portion, and, after a measured period of time,produces a second series of clock pulses that causes the ADC means tosample said subsequent decaying ramp portion.
 7. X-ray fluorescenceapparatus as set forth in claim 4, wherein said amplifier means includesa preamplifier, rejection means for unwanted high frequency signals andan amplifier.
 8. X-ray fluorescence apparatus as set forth in claim 4,and including means for determining the slope of said first decayingramp.
 9. X-ray fluorescence spectroscopy apparatus as set forth in claim4, wherein said digital processing means includes means for determiningthe amplitude difference values between said extension of said firstdecaying ramp and said next subsequent decaying ramp at a plurality ofpoints corresponding to the sample points and developing the averagevalue thereof.
 10. The method as set forth in claim 1, wherein theemission spectroscopy system is an X-ray fluorescence system.
 11. Themethod as set forth in claim 3, wherein the emission spectroscopy systemis an X-ray fluorescence system.